An increasing interest has been shown in recent years in the fractionation of paper pulp fibres according to size and shape of the fibres in aqueous suspension. By fractionation of groundwood pulp, for example, different grades of pulp can be obtained to meet the needs of various paper grades or paper machinery. Moreover, fractionation can enable a more efficient and more economic treatment of one fraction for enhancement of its quality without damage of the quality of another fraction. Fractionation can also be employed to even out stock quality fluctuations and would be an alternative to the large blending tanks forming a conventional part of all groundwood mills.
Although the above benefits of fractionation have been known in the paper industry for a long time, no equipment for performing fractionation has hitherto been successfully put into practice. One known method for the fractionation of groundwood pulp using a centrifugal screen has appeared to be efficient for removing a coarse fibre fraction amounting to 10 to 30 per cent of the pulp weight, but is not suitable for dividing groundwood pulp into fractions containing essentially long fibre and fines, respectively. Moreover, the expense of the process in terms of high energy consumption and severe water pollution has been considered too high to justify common industrial use of the process.
The growing pressure for recycling waste products and the use of so-called secondary fibres have made the paper industry look more closely at improved fractionation methods. By fractionating a mixed waste paper pulp, a rearrangement of the different constituents of the raw material can be obtained, whereby the pulp can be separated into fractions containing higher amounts of chemical fibres or groundwood fibres, respectively. Moreover, by fractionating a more uniform waste such as waste corrugated board, the quality of the long fibre fraction will be better than that of the raw material, and refined pulp obtained by such a fractionation may be used for purposes where more expensive pulps are conventionally employed.
The industrial requirements described above have resulted, inter alia, in the so-called Johnson-fractionator which has been described e.g. in the article "Fractionation of Fiber Suspension by Liquid Column Flow" by Gunnar Olgard in the periodical TAPPI, Vol. 53, No. 7, July 1970. In the Johnson fractionator, an assembly of tubes open at each end is used, and the fractionator works with so-called "plugs", i.e. volumes of liquid with suspended matter which are contained between air bubbles. The fractionation is mainly due to two phenomena taking place within such a plug. In pulp flow the wall shear forces will tend to collect the larger particles in the center of the tube. Furthermore, there will be an internal flow pattern in each plug, relative to the mean velocity, there will be a forward flow in the center and a backflow in the wall region.
As a result thereof, the bigger particles will tend to be transported to the front end of the plug, and each plug can be fractionated by separating the front and rear ends at the ejection end of the tubes.
However, the Johnson fractionator has not been successful in practice mainly due to the fact that to secure satisfactory fractionation, the cross section of each tube has to decrease towards the ejection end, whereby the tubes may easily become blocked and prevent the flow of the plugs. Although the operation costs of the Johnson fractionator are not very high, this apparatus still suffers from the same disadvantage as the older centrifugal screen arrangement that only suspensions of a very low concentration can be handled.